Hi,

I'm searching a bacteria strain without a recombinant action and able to growth with a plasmid which express a toxic protein for the bacteria. I transformed Dh5alp and BL21 but those two one cut or recombine this plasmid (11Kb,code for a viral polyprotein toxic for bacteria).

Thanks a lot

You might try the STBL2 or SURE strains. Invitrogen and Stratagene.

Are you sure your problem is recombination, and not simply a very strong selection against your insert? William Studier has done a great deal of study on leaky expression of proteins caused by trace levels of lactose in media.

I would suggest you transform your cells and use SOC and a glucose-containing agar medium to recover clones. In our experience, colonies grow to several times their "usual" size, showing that even supposedly benign inserts can have a large effect on the cells.

If this is the problem, you'll have to grow the cells to a high OD before you do a short induction. Again, you'll need to use a medium that totally represses T7 polymerase expression (I am assuming you're using a pET-based expression system...).

Whatever the cause of your problem, good luck! You might have to move to another expression system, like yeast, or in vitro transcription/translation.